1. Field of the Invention
The present invention relates to a method of electrodeposition coating that hardly generates poor appearance in coating an object to be coated, particularly a zinc steel plate. The method uses a lead-free cationic electrodeposition coating composition with high throwing power and enables electrodeposition coating in short time.
2. Description of the Background Art
Electrodeposition coating method can perform coating of an object to be coated into details and can perform the coating automatically and continuously even if the object to be coated has a complex shape. Therefore, the electrodeposition coating method is widely used as an undercoating method for an object to be coated that has a large and complex shape and needs high rust prevention, such as an automobile body. Further, as compared with other coating methods, the efficiency of using a coating composition is extremely high, so that the method is economical. Therefore, the electrodeposition coating method is widely spread as an industrial coating method. The cationic electrodeposition coating method is carried out by immersing an object to be coated into a cationic electrodeposition coating composition as a cathode, and applying a voltage thereto.
Hitherto, lead has been added to an electrodeposition coating composition in order to improve the corrosion resistance of the coated film. In recent years, there is a demand for reduction of lead that is used in an electrodeposition coating composition because lead gives adverse effects on the environment.
In the meantime, there is a desire for reduction of the amount of use of the coating composition itself in order to decrease the coating costs.
Deposition of coated layer in the process of cationic electrodeposition coating owes to electrochemical reaction, and the coated layer is deposited on a surface of the object to be coated by application of voltage. Since the deposited coated layer has a dielectric property, the electric resistance of the coated layer will increase according as the deposited layer increases in thickness by progression of the deposition of the coated layer during the coating process.
As a result of this, deposition of the coating composition onto said sites decreases and, instead of this, deposition of the coated layer onto noncoated sites starts. In this manner, the solid component of the coating composition is successively deposited to the sites to be coated, thereby completing the coating. In the specification of this application, the property by which the coated layer is successively formed onto non-coated sites of the object to be coated is referred to as “throwing power”.
In the cationic electrodeposition coating method, a dielectric coated layer is successively formed onto the surface of the object to be coated as described above, so that the coating composition theoretically has an infinite throwing power that makes it possible to form the coated layer uniformly over the entire surface of the object to be coated.
However, at the non-coated sites of the object to be coated, the voltage applied in the bath becomes weak as compared with the coated sites, so that the solid component of the coating composition hardly attaches to the non-coated sites, therefore the throwing power of the electrodeposition coating composition is not necessarily sufficient, thereby generating uneven layer thickness.
Cationic electrodeposition coating is normally used for undercoating, and a major object thereof is rust prevention or the like, so that the thickness of the coated film on all sites must be above a predetermined value even if the object to be coated has a complex structure. For this reason, if the layer thickness is uneven, the thick parts are coated too much, results in excessive use of the coating composition. Therefore, in order to reduce the amount of use of the coating composition, one must improve the throwing power of the electrodeposition coating composition.
Steel plates that has been heretofore used in an automobile body occupy major parts of the object to be coated for cationic electrodeposition coating. Steel plates are stored by being coated with a rust-preventing oil. Usually, steel plates that has undergone removing this rust-preventing oil with alkali or the like and subjected to surface treatment, are used as an object to be coated.
However, in recent years, electrodeposition coating is often carried out on zinc steel plates in which the surface of steel plates have been plated with zinc. The zinc steel plates are excellent in rust-preventing property as compared with normal steel plates, so that the zinc steel plates can realize an enhanced rust-preventing property if they are used as an object to be coated. On the other hand, if zinc steel plates are used as an object to be coated, pinholes or craters are liable to be generated in the obtained electrodeposition coated layer, thereby a problem in that poor appearance is likely to be generated. The reason for that is considered to be due to facilitated generation of spark discharge in hydrogen gas, because the discharge voltage of hydrogen gas generated on the object to be coated side at the time of cationic electrodeposition coating is lower in the zinc steel plates than in the iron steel plates.
In addition, in recent years, there is a demand for short-time electrodeposition coating in order to improve the productivity. However, short electrodeposition time raises a problem of decrease in the throwing power.